I used it for a while about a year ago.
I live in Rockwall and found the service there, at the time at least, to be very poor. Routinely dropped calls.
The program was as you would expect - I didn't find any tricks or catches.
The BIGGEST issue I had is with exiting the program. We went back to Cricket (which is where we had been prior to trying Fi) and they needed my account # to transfer the service. This is typical and you normally do this, then Cricket sends you a SIM and when you put it in the phone, it transitions the account, nice and easy. Well, Google FI wouldn't give me the account number until I canceled. They told me that upon canceling, service would be cut immediately. This meant I'd have 2 days or so without service while waiting for the SIM to come in the mail. Luckily, I had a Cricket brick and mortar store near by, so I went there and they got me set up a few hours after I canceled. Also, Google actually DIDN'T cancel the service immediately, so I may have just been on the end of bad communication from a customer service rep (though I did have screen shots verifying they told me that would happen).
I would have stayed with them had the coverage been good, but at this point Cricket has upped their data per user and they're still cheaper - at least that FI was at the time.
That said, I would give them a try if you're looking - just wasn't right for us.
And I brought a Nexus 6P when I joined, so not sure on the device questions.